White said Struve was warming up backstage before the fight, as all fighters do, when he started to feel off. White said the doctors reported Struve had an elevated heart rate and started hyperventilating.

"Stefan Struve suffered a non-life-threatening, near-fainting spell backstage," the UFC said in a statement released during the pay-per-view portion of the card. "Afterwards, the medical team did not feel he was fit to compete. With his health and safety in mind, he’s been removed from the card and is currently under the care of the medical staff."

Struve, a 7-foot heavyweight from the Netherlands, was making his return to the UFC after being diagnosed with a leaking aortic valve and an enlarged heart last August. Struve last fought on March 3, 2013, losing by third-round knockout to Mark Hunt.

"They think it was more of a panic attack," White said. "We don't want to risk it."

The news broke during the first fight of pay-per-view between Marcus Brimage and Russell Doane. The Struve-Mitrione fight was the third bout of the pay-per-view leading into Ronda Rousey vs. Alexis Davis and Chris Weidman vs. Lyoto Machida.

“I found out what happened and went to talk to Stefan." Mitrione said. "He was still in his chair and was very apologetic. He’s such a competitor. Obviously health and safety come first.”